The present invention relates to keel joint centralizers for a tension leg platform (TLP) for testing and producing hydrocarbon formations in offshore waters.
Traditional TLPs having a four-column construction, include a four column semi-submersible floating substructure, multiple vertical tendons attached at each corner, tendon anchors to the seabed, and production risers. The TLP production deck is supported above the water surface by four columns that pierce the water plane. These types of TLPs typically bring a well(s) to the surface for completion and are meant to support from 20 to 60 wells at a single surface location. The production risers are restrained at the production deck and at the seabed. Restraint of the production risers in this manner allows environmental loading to move the risers considerable distances and requires large spacing between risers at the production deck to prevent riser interference.
Traditional solutions to guiding risers have utilized elastomeric joints, ball joints, and steel centralizers. These solutions have been used on Spars that are restrained to the seabed using mooring lines. TLPs, however, are connected to the ocean floor by rigid tendons, so the motions are smaller and a TLP hull is not typically as deep as a Spar hull. Spar hulls do not typically allow the use of external tieback connectors, which require an opening of at least 50 inches diameter. The present invention allows full passage of external tieback connectors, and is still compatible with internal tieback connectors having a smaller outside diameter.
In a mono-column TLP it is desirable to keep well bay spacing to a minimum, and to keep the hull diameter to a minimum. Therefore the production risers must be restrained at the lower end of the hull. Applying restraint to the production risers at the lower end of the hull produces an increase in bending stresses at the point of restraint. A common practice on subsea risers for controlling bending stresses has been the use of tapered riser keel joints to distribute the load over a sufficiently long section of the riser joint.
Some problems associated with previous keel joint riser centralizers include high cost and excessive friction forces applied to the TLP's hull. In addition, use of elastomeric concepts is very difficult to analyze and quantify their useful life. Previously used concepts on Spars have relied on a steel-to-steel interface, which is subject to corrosion, galling, high friction forces and requires a large size.
It is therefore an object of the present invention to provide a riser keel joint centralizer for transferring lateral loads from the riser to the TLP hull.
It is another object of the present invention to provide a riser keel joint centralizer having a radiused peripheral profile for preventing binding of the keel joint centralizer during riser and TLP motions.
It is yet another object of the present invention to provide a riser keel joint centralizer utilizing a non-metallic composite bearing material for minimizing contact stresses at the working surfaces of the keel centralizer.
It is still another object of the present invention to provide a riser keel joint centralizer including corrosion resistant properties.
It is still another object of the present invention to provide a riser keel joint centralizer for accommodating angular offset of a riser relative to a keel guide sleeve.
It is still another object of the present invention to provide a riser keel joint centralizer generating low friction without stick-slip characteristics at the riser to platform hull interface.